1. Technical Field of the Invention
The present invention relates generally to a power supply control system for a heater working to heat a gas sensor such as a gas concentration sensor up to a desired activation temperature which may be employed in an air-fuel ratio control system for automotive vehicles for measuring the concentration of a specified component such as O2, NOx, or CO contained in exhaust emissions from the engine.
2. Background Art
Air-fuel ratio control for automotive internal combustion engines is typically accomplished using an output of a gas concentration sensor. Such a gas concentration sensor has a sensor element which includes a solid electrolyte member made of zirconia. The sensor element works to measure the concentration of a given gas component (e.g., oxygen) of exhaust gasses of the engine. An air-fuel ratio control system determines an air-fuel ratio as a function of the measured concentration of the gas component. Ensuring the accuracy of such a determination requires keeping the sensor element at a desired activation temperature. This is usually achieved using a heater embedded in the sensor element. The amount of heat generated by the heater is regulated, for example, by changing the duty cycle of a pulse signal used to switching on and off a power supply to the heater. A feedback control system is proposed which measures the resistance of the sensor element and achieves the regulation of the power supply by changing the duty cycle of the pulse signal to bring the measured resistance to agreement with a target one.
The gas concentration sensor usually undergoes an undesirable change in power supply due to a drop in voltage applied to the heater through a harness and/or a change in output voltage of a power source. This results in lowered controllability of the power supply to the heater.
Further, switching between on-time and off-time of the duty cycle of the pulse signal usually cause electric noises to be added to the harness coupled to the heater. Therefore, if such switching overlaps with a period of time during which the resistance of the sensor element is measured, it may result in an error in measuring the resistance of the sensor element. This problem is encountered frequently, especially in a case where harnesses leading to the sensor element and to the heater are tied together.